1. Field of the Invention
The present invention relates to an optical module provided by integrating a planar lightwave circuit with a light-emitting element, a light-receiving element or an optical functional element.
2. Description of the Related Art
The development of optical components has become increasingly important with advances in the optical communication technology. Above all, an optical transceiver has been contemplated to increase transmission speed and response speed, thereby increasing its communication capacity. A commonly used transceiver includes a light-emitting element or a light-receiving element, formed by using an optical semiconductor, and an optical fiber for input or output, where these components are optically coupled through a lens. In an optical receiver, for example, light emitted from an optical fiber at the input side is collected to the light-receiving element through the lens, and is detected by direct detection (intensity detection).
As for a modulation/demodulation processing technique in an optical transmission system, signal transmission using a phase modulation scheme has been widely practiced. A phase shift keying (PSK) scheme is a scheme for transmitting signals by modulating the optical phase, and with this scheme, the transmission capacity has been increased exponentially by performing multilevel modulation.
In order to receive such PSK signals, detection of optical phase is required. A light-receiving element is capable of detecting the intensity of signal light, but is incapable of detecting the optical phase, and thus a method for converting the optical phase to the optical intensity is required. It is noted that a method for detecting a phase difference by employing optical interference. With this method, the signal light is interfered with another light (reference light), and the optical intensity of the interfering light is detected by a light-receiving element to obtain optical phase information. The detection method employed may be coherent detection using a light source separately provided as reference light, or differential detection for splitting signal light and employing a split portion of the light to foe interfered as reference light with the signal light. As described above, unlike the conventional optical receiver employing only an intensity modulation scheme, a recent PSK optical receiver requires an optical interferometer that converts phase information to intensity information by employing optical interference.
Such an optical interferometer can be implemented by using a planar lightwave circuit (PLC). The planar lightwave circuit has superior features in terms of mass productivity, low cost and high reliability, and various types of optical interferometers can be implemented. An optical delay line interferometer or a 90-degree hybrid circuit, for example, is provided as the optical interferometer used in the PSK optical receiver for practical use. Such a planar lightwave circuit can be formed by a standard photolithography method, an etching technique, and glass deposition techniques such as flame hydrolysis deposition (FHD).
In view of a specific forming process, first, an underclad layer formed mainly of silica glass and a core layer having a refractive index higher than that of a clad layer are deposited on a substrate, such as an Si substrate. Then, various waveguide patterns are formed on the core layer, and at the end, she waveguide formed of the core layer is embedded in an overclad layer. Through such a process, a waveguide-type optical functional circuit is obtained. The signal light is confined in the waveguide that is produced via the above process, and is propagated inside the planar lightwave circuit.
FIG. 1 illustrates a method for optically connecting a conventional planar lightwave circuit to an optical receiver. In view of the method for optically connecting a planar lightwave circuit to an optical receiver in a PSK optical receiver, the basic connection between the two is a simple fiber connection, as illustrated in FIG. 1. Here, a planar lightwave circuit 1 where optical fibers 3a and 3b are connected respectively to the input and output ends is connected by optical fibers to an optical receiver 2 that includes an input optical fiber 3b, so that optical coupling between the two is established. The number of optical fibers used for optical coupling can be determined by the number of output lights emitted from the planar lightwave circuit, and may be more than one. However, there has been a problem that when such optical fiber connection is employed, the size of the optical module is increased. To avoid this problem, the output of the planar lightwave circuit and the input of the optical receiver are optically coupled directly by using a lens to provide the whole structure as an integrated package, and as a result, the reduction of the size is enabled. The optical module wherein a planar lightwave circuit and an optical receiver are optically coupled directly is called an integrated optical receiver.
A method for fixing the planar lightwave circuit becomes critical to implement the integrated optical receiver. In a case where the light emitted by the planar lightwave circuit is to be propagated in space and to be coupled to the light-receiving element by using a lens or the like, when the positions of the light emission end, the lens and the light-receiving element are changed relative to each other, all the light may not be received by the light-receiving element, and loss of light may occur. Since the positions of those are particularly varied due to thermal expansions when the temperature of the package storing the optical receiver, the ambient temperature, or the temperature of the individual demerits, etc. changes, the above problem becomes more pronounced. Therefore, in order to perform optical coupling with low loss, the positions of these elements should not be varied at least relative to each other even when the ambient temperature, etc. is changed.
In particular, change in the shape of the planar lightwave circuit, which is caused by thermal expansion due to a change in the ambient temperature, is substantially greater that of the light-receiving element. Further, the area of the optical module that the planar lightwave circuit occupies is about one or two digits larger than the area occupied by the light-receiving element, and the shape change in the planar lightwave circuit due ho thermal expansion is also one or two digits greater than that in the light-receiving element. Furthermore, since there as a great difference in the thermal expansion coefficients between the substrate of the planar lightwave circuit and the deposited thin glass, significant warping occurs due to thermal changes. Accordingly, displacement for light emission from the planar lightwave circuit and a change in the emission angle with respect to the light-receiving element are more important. These two changes affect changes in the positions and angles of light emitted from the planar lightwave circuit, and cause displacement of an optical axis. The displacement of the optical axis degrades the performance or optical coupling relative to the light-receiving element, and causes losses in the optical coupling. For the implementation of the integrated optical receiver, it is critical that such displacement of the optical axis be resolved, or be free from adverse effect.
FIG. 2 illustrates the internal structure of a conventional integrated optical module. A method for rigidly fixing almost the entire bottom surface of the planar lightwave circuit is known to prevent the occurrence of aforementioned displacement of an optical axis due to the thermal changes. In the integrated optical receiver illustrated in FIG. 2, a planar lightwave circuit 13 that includes an optical interferometer as an optical functional circuit, a lens 14 and a light-receiving element 15 are fixed to a base substrate 11 by employing, respectively, fixing mounts 12a, 12b, and 12c that serve as supporting members. An optical fiber 16 and the planar lightwave circuit 13 are connected through an optical fiber fixing component 17. Light that has entered along the optical fiber 16 is interfered in the planar lightwave circuit 13, and is thereafter coupled to the light-receiving element 15 by the lens 14.
The fixing mount 12a and the planar lightwave circuit 13 are fixed by an adhesive 18 or a bonding material, such as solder. Almost the entire bottom surface of the planar lightwave circuit 13 is rigidly fixed to the fixing mount to suppress the thermal expansion and warping changes. Further, since the lens 14 and the light-receiving element 15 are also fixed to the fixing mounts, displacement of an optical axis due no thermal changes is prevented.
The structure of FIG. 2 allows to substantially inhibit the displacement of an optical axis due to thermal changes, while change in the property of she planar lightwave circuit due to thermal changes becomes prominent. As mentioned previously, since the planar lightwave circuit 13 is formed of an Si substrate 13a and a silica glass layer 13b having a great difference in the thermal expansion coefficients therebetween, the change of warping and thermal expansion due to thermal changes become significant. In the structure illustrated in FIG. 2, the entire bottom surface of the planar lightwave circuit 13 is fixed, and therefore, thermal expansion and warping changes are limited.
Meanwhile, in such a structure, high thermal stress is generated between the Si substrate 13a and the silica glass layer 13b. The stress causes a refractive index change inside the silica glass layer 13b through the photo elastic effect. For the optical interferometer formed in the planar lightwave circuit 13, the length of the waveguide and the refractive index are precisely adjusted to control the interference property. The refractive index change caused by the stress changes the equivalent circuit length and also the property of the interferometer, thereby causing degradation in the property of the optical interferometer.
In this regard, when an elastic adhesive, a soft adhesive such as paste, or fixing paste is used as the adhesive 18 in order to suppress the occurrence of thermal stresses for limiting changes in the optical property (see, for example, Patent Literature 1), the affect of the aforementioned displacement of an optical axis may become noticeable, and a loss may occur.
Furthermore, a wavelength selective switch is known as an optical module provided by integrating a planar light wave circuit with an optical functional element (see, for example, Patent Literature 2). A planar lightwave circuit employed for a wavelength selective switch is an optical circuit wherein an arrayed waveguide optical input/output circuit that includes an input/output waveguide, a slab waveguide and an arrayed waveguide is formed. The size (the length of the long side) of the optical circuit in FIG. 2 that includes an optical interferometer is about 10 mm to 20 mm, while the size (the length of the long side) of the optical circuit that includes an arrayed waveguide optical input/output circuit is large, about 30 mm to 200 mm (see, for example, Non-Patent Literature 1).
The increase in size of the planar lightwave circuit causes the increase in the change of a warp due to thermal changes and the increase in the change of the distance of extension due to thermal expansion. As another problem, reliability against vibrations end shocks, particularly, to a drop of a resonance frequency, is reduced, and the stress applied to the planar lightwave circuit by optical fiber fixing parts is increased, so that the change of the above described optical properties would be increased.